Marine counter-rotating shaft drive mechanism

ABSTRACT

A Counter-Rotating Shaft Drive Mechanism for a boat comprises an engine gearbox output shaft coupling ( 2 ); a differential arrangement which creates counter-rotating outputs in a dual output shaft arrangement and comprises a housing ( 1 ) and removable maintenance plates ( 4 ), means for arresting rotation of the differential housing and a coupling between the shaft arrangement and a twin propeller arrangement. A single drive gear ( 8 ) may be used to drive the output shaft arrangement which may comprise a pair of concentrically arranged shafts ( 5 ) and ( 9 ) which rotate in opposite directions. The inner and outer shafts may have rubbing pads arranged between them, while the shaft arrangement may operate as s self-lubricating sealed unit. The housing, maintenance plates, and shafts may be manufactured from a metallic material, a plastic polymer or a carbon composite. The Marine Counter-Rotating Shaft Drive Mechanism has been designed to be both retrofitable, and factory fit.

This invention relates to a Marine Counter-Rotating Shaft DriveMechanism.

BACKGROUND

Counter-rotating propellers eliminate waste. In this type ofinstallation, two propellers are positioned one immediately ahead of theother on the same shaft line, but rotating in opposite directions. Therotational energy imparted to the water by the forward of the twopropellers is cancelled out by the opposite rotation of the rearwardpropeller. The slipstream from counter-rotating propellers is almostsmooth and straight, with little twist. When a single propelleraccelerates water into itself from ahead and expels it astern, itgenerates thrust like a jet engine. Unfortunately, a significantpercentage of the power delivered to the propeller also goes to twistingthe water around, creating the helically shaped propeller wake. Thisenergy is exhausted and does nothing in providing propulsion, or drive;it is, simply, wasted. Counter-rotating propellers dramatically reducethis waste effect and are 10 to 30 percent, dependant on configuration,more efficient than standard, single propellers.

As a direct result of improved efficiency, this counter-rotatingpropeller arrangement offers improved thrust, handling, fuel efficiencyand smoothness of operation over a single propeller.

This invention relates to a counter-rotating shaft drive application.For boat builders—the main, but not sole, target for thisapplication—the decision between shaft drives or sterndrives as a drivetrain is a dilemma.

Shaft drives are more conventional, and in the simplest sense use ashaft to transmit power from the engine's gearbox output shaft through aseal, or gland, in the boat's hull to the propeller. This propeller sitsforward of the steering gear. With shaft drives, engines are typicallymounted towards the centre of the boat, although rearward mountedapplications using a ‘Z’ drive arrangement are in use, and separatestern gear provides steerage.

Sterndrives, on the other hand, essentially combine the engine's gearboxand propeller running gear in a single unit, known as a ‘sterndrive’.With this installation, engines are mounted at the stern of the boat andthere is no separate drive shaft arrangement. It is normal for thesterndrives to vector (turn) in order to provide steerage. Sterndriveapplications are less costly as a lower power engine can be used due toincreased efficiency of propulsion.

For boat builders with an overall craft length of below 40 ft (12 m),the choice of power train is relatively easy—diesel engines coupled tosterndrives can provide the necessary thrust to achieve the 30-knot holygrail that today's market demands. Above 40 ft (12 m), the choice isalso clear—the 300 horsepower ceiling imposed by the mechanicallimitations of current sterndrives dictates a move towards shaft drivessince more power is required to achieve the magic 30 knots. But between35 ft (10.7 m) and 45 ft (13.7 m), the current trend is shifting towardssterndrives because the boat builder does not have to contend withexhausts, steering gear, P-brackets, propeller shafts and alignmentissues. Sterndrives are easier and cheaper to install and are inherentlymore power efficient than a conventional shaft drive propeller because,compared with a streamlined stern drive, the drag generated by thepropeller shafts, rudders and P-brackets is significant. This all helpsin achieving the magic 30 knots. However, the boat-buying public areremarkably pro shaft drives, favouring their inherent durability.

High-power sterndrives equipped with counter-rotating propellers arecommercially available. The mechanical construction of sterndrives makesfor a readily adaptable counter-rotating installation due to short shaftlengths and close-quarter gearing. The inherent drawback of this type ofarrangement is complexity, and general vulnerability given that the bulkof the drive gear is exposed under the waterline at the stern of thecraft. Shaft drives, on the other hand, are by their nature moredurable.

-   -   According to the present invention, titled Marine        Counter-Rotating Shaft Drive Mechanism, there is provided a        methodology of combining sterndrive performance with shaft drive        installations. The invention provides a method of creating        counter-rotating output from a single input and is designed to        be both retrofitable and factory fit.

Sterndrives will not endure neglect as well as shaft drives and do notoffer the same lateral grip on water. Whilst sterndrives offer boatsincreased manoeuvrability due to their vectoring (turning of thesterndrive with the helm wheel), it is this vectoring that makessterndrive-equipped boats heel more in turns because part of the thrustproduced by the vectored drives is trying to turn the boat over. Thereremain many merits, therefore, to shaft drives.

The Marine Counter-Rotating Shaft Drive is designed to bring the thrustand efficiency of sterndrives to shaft drive boats with the addedbenefit of increased durability, reliability and stability oversterndrives. Overall, a significant performance gain for conventionalshaft drive installations is the end result. Sterndrives are notretrofitable to shaft drive boats. The Counter-Rotating Shaft Drive hasbeen designed to be both retrofitable, and factory fit.

In summary, The Marine Counter-Rotating Shaft Drive Mechanism is aself-contained, maintainable sealed mechanism that is both suitable forfactory fit and retrofit applications. Basically, it is designed to slotin between the recipient craft's gearbox output shaft and its driveshaftthrough-hull gland. After substituting the recipient craft's driveshaftwith a shorter dual shaft arrangement, the Counter-Rotating Shaft DriveMechanism slots into place (the shorter drive shafts allow for themechanism to be accommodated without moving engine installation or glandfittings). The invention is very much aimed at motorboats in the 12 mrange as opposed to deep draft ships.

TECHNICAL FEATURES

According to the present invention, there is provided a MarineCounter-Rotating Shaft Drive Mechanism comprising: an engine gearboxoutput shaft coupling; differential arrangement creatingcounter-rotating outputs in the form of a housing and removablemaintenance plates; a combined, dual, concentric shaft arrangementallowing for counter rotation; means for arresting rotation of thedifferential housing, and a coupling to a twin propeller arrangementsited at the propeller end of the dual shaft arrangement.

A specific embodiment of the invention will now be described by way ofan example with reference to the accompanying drawing in which:

FIG. 1 shows a schematic of the arrangement, side-on view

FIG. 2 shows an end-on view of FIG. 1

FIG. 3 shows the principle of the counter-rotating shaft's rubbing padarrangement.

FIGS. 1 and 2

Referring to the drawing FIG. 1, the Marine Counter-Rotating Drive ShaftMechanism comprises a differential housing 1 linked to, but notmechanically fixed to, the gearbox output shaft 2. To provideflexibility of installation, a range of adapters 11 may be insertedbetween 1 and 2. A coupling to a twin propeller arrangement 14 is sitedat the propeller end of the dual shaft arrangement.

The differential housing 1 comprises a machined body designed to carryopposing bevel idler gears (3). The differential housing 1 is arrestedthrough the use of anti-rotating tie bars 10. This enables the internalpower train to operate, driven by the engine's gearbox output shaft 2.Initial shock loads caused by engagement of the gearbox output shaft areabsorbed by rubber mountings 7. Two side plates 4, which are removable,are used to retain the opposing idler bevel gears 3 in place. The bevelgears 3 are driven by a single bevel idler gear 8 secured to theengine's gearbox output shaft through the use of a spline and key orsimilar. The driver bevel gear 8 and idler bevel gears 3 are used todrive the outer drive shaft 5 through a driven bevel gear 6. The bevelgear set is designed to use either straight or helical gear teeth forefficient power transfer. The gearing arrangement 8,3,6 reverses thedirection of the input drive from the engine's gearbox output shaft 2thereby providing counter rotation. The gearing arrangement ismaintained through removal of the side plates 4 and end plates 12. Thedrive shafts 5 and 9 are concentric to each other.

The driven bevel gear 6 is attached mechanically to the outer outputshaft 5 through the use of a spline and key or similar.

The inner drive shaft 9 is secured mechanically to the driving gear 8.This mechanical fixing dictates that the inner drive shaft 9 is drivenin the same direction as the engine's gearbox output shaft 2. Thisdirection of drive is opposite to that of the outer drive shaft 5 andcounter rotation is achieved. Mechanical securing of the inner driveshaft to the driver bevel gear 8 is achieved through a spline and key orsimilar. The differential arrangement shown in FIG. 1 runs as a sealedunit. The invention has been scoped to manufacture the differentialhousing 1, side plates 4, end plates 12, outer drive shaft 5 and innerdrive shaft 9 from either a metallic material or that of carboncomposite. Choice of material used within the construction of theCounter Rotating Shaft Drive Mechanism is dependent on application.Larger installations will benefit from the weight saving properties ofcarbon composite.

FIG. 2

The outer drive shaft 5 is comparable in size to the recipient craftmanufacturer's original drive shaft specification. The inner driveshaft's 9 rotation is counter to the outer shaft 5. Drive shafts 5 and 9are concentrically arranged.

FIG. 3

To ensure optimum performance, the counter-rotating shaft arrangementmakes use of bearings at either end and bespoke rubbing pads 13positioned along the shaft's length. A key feature of these pads istheir irregular spacing. This prevents ‘bounce’ and vibration of theinner shaft. The number of rubbing pads 13 is dependent on the length ofthe outer drive shaft 5. The rubbing pads are manufactured from aself-lubricating material and are located within pockets on the innerdrive shaft 9. The shaft arrangement is sealed either end through theuse of dual seals at each end.

PROPELLER ARRANGEMENT

The invention has been scoped to adopt a propeller arrangement involvinga propeller of smaller diameter and more blades behind a propeller oflarger diameter and fewer blades. This is because the rearward propelleris working in a faster water flow than the forward propeller and, as aresult, it must have a smaller diameter and steeper pitch. Bothpropellers are designed to absorb the same horsepower—hence theadditional blade on the rearward propeller.

Because of the additional blades in total, a counter-rotating propellersystem has more blade area and therefore lower blade loading than acomparable single propeller at the same horsepower. This will reducecavitation (bubbles of partial vacuum caused by excessive propellerspeed or loading) problems, and offer a marked improvement in vibration.

-   -   The Marine Counter-Rotating Shaft Drive is designed to bring the        thrust and efficiency of sterndrives to shaft drive boats with        the added benefit of increased durability, reliabiity and        stability over sterndrives. The Counter-Rotating Shaft Drive        Mechanism has been designed to be both retrofitable, and factory        fit.

1. A Counter-Rotating Shaft Drive Mechanism comprising: an enginegearbox output shaft coupling; differential arrangement creatingcounter-rotating outputs in the form of a housing and removablemaintenance plates; a combined, dual, concentric shaft arrangementallowing for counter rotation; means for arresting rotation of thedifferential housing, and a coupling to a twin propeller arrangementsited at the propeller end of the dual shaft arrangement.
 2. ACounter-Rotating Shaft Drive Mechanism as claimed in claim 1 wherein asingle drive gear is used to rotate the two separate propeller shafts inopposing directions.
 3. A Counter-Rotating Shaft Drive Mechanism asclaimed in claim 2 wherein the single drive gear is provided as theinput to the differential housing.
 4. A Counter-Rotating Shaft DriveMechanism as claimed in claim 1, 2 or 3 wherein the two concentric driveshafts operate on the same shaft line, but rotate in oppositedirections.
 5. A Counter-Rotating Shaft Drive Mechanism as claimed inany preceding claim wherein the drive train is maintainable through theremovable maintenance plates.
 6. A Counter-Rotating Shaft DriveMechanism as claimed in any preceding claim wherein self-lubricatingrubbing pads are used to support the inner shaft within the outer shaft.7. A Counter-Rotating Shaft Drive Mechanism as claimed in any precedingclaim wherein the dual, concentric shaft arrangement is provided with aleast one set of rubbing pads located on the inner drive shaft.
 8. ACounter-Rotating Shaft Drive Mechanism as claimed in claim 6 whereinthe, or each, rubbing pad set is spaced unevenly around thecircumference of the inner shaft to provide smoother counter-rotatingmotion between the two shafts.
 9. A Counter-Rotating Shaft DriveMechanism as claimed in any preceding claim wherein the dual shaftarrangement operates as a self-lubricating sealed unit.
 10. ACounter-Rotating Shaft Drive Mechanism as claimed in any preceding claimwherein the differential housing, maintenance plates, outer drive shaftand inner drive shaft are manufactured from either a metallic material,plastic polymer or carbon composite.
 11. A Counter-Rotating Shaft DriveMechanism as claimed in any preceding claim wherein the dual,counter-rotating, drive shaft arrangement is designed for water andcontaminant exclusion.
 12. A Counter-Rotating Shaft Drive Mechanism asclaimed in any preceding claim wherein the differential housing is heldbetween the gearbox output flange and the drive shafts.
 13. ACounter-Rotating Shaft Drive Mechanism substantially as described hereinwith reference to FIGS. 1-3 of the accompanying drawing.